The present invention relates to a process for controlling the degree of polymerization of vinyl polymers. The term "vinyl polymers" as herein used includes both vinyl homopolymers and vinyl copolymers.
It is well known that where vinyl monomers are polymerized in the presence of a conventional radical initiator, polymers having a high degree of polymerization can be obtained.
Particularly, in the case of acrylic monomers, polymers having a considerably high melt viscosity are obtained. Thus, these polymers are accompanied by problems such as difficulty in molding, high temperature required for the molding, coloring or decomposition by heating of moldings of the polymers, which give rise to deterioration in quality, and furthermore, difficulty of carrying out a long molding operation.
Thus, in order to produce polymers having good workability, methods for controlling the degree of polymerization of the polymers to lower levels, for example, by polymerizing at high temperatures, or by adding a chain transfer agent, have been employed.
In the method in which the polymerization temperature is raised, the degree of polymerization cannot be controlled as desired, and furthermore, polymers having branched chains are produced because of the high polymerization temperature, which results in reduction of heat stability in the case of some kinds of monomers used. Thus, this method is not desirable.
In the method comprising adding a chain transfer agent, hitherto known chain transfer agents have various defects, and satisfactory ones have not been found. For example, as effective chain transfer agents for the polymerization of vinyl monomers, halogenated hydrocarbons, halogenated carbons, amines, mercaptans, hydroquinones, unsaturated cyclic terpenes, and the like are known. However, all these compounds are accompanied by problems.
In the polymerization of vinyl monomers, carbon tetrabromide, which is a typical example of the halogenated carbons, is an excellent chain transfer agent and is capable of providing vinyl polymers having low degrees of polymerization. However, the vinyl polymers obtained are extremely inferior in heat stability and are considerably colored at the molding stage, thus being of low practicality. Of the mercaptans, low molecular weight mercaptans containing, for example, 2 or 4 carbon atoms are highly effective and are capable of providing low molecular weight vinyl polymers without causing reduction of heat stability as caused with the carbon tetrabromide. However, the vinyl polymers obtained have a considerably unpleasant odor and retard the polymerization reaction. Also, mercaptans containing 8 or more carbon atoms are effective chain transfer agents, but the vinyl polymers obtained have unpleasant odor, and unreacted mercaptans are difficult to remove because the boiling point thereof is high. Thus, these vinyl polymers are not suitable for use in a material for packages, containers, or the like for foodstuff and are limited in use to an extremely narrow field.
Moreover, the amines, hydroquinones, and unsaturated cyclic terpenes have a fatal drawback in that they retard or stop the polymerization reaction although they are effective chain transfer agents.
In general, as the chain transfer agent, those compounds which contain atoms that easily cause an extraction reaction or the like to move a radical and which stabilize a polymer radical, for example, a hydrogen atom, a halogen atom, and the like are used. With these compounds, it is required that the bond energy of the hydrogen atom or the like be of an order such that the hydrogen atom or the like is easily extracted by a polymer radical and that a chain transfer agent radical formed by the above extraction reaction has a property to initiate again the polymerization reaction.
Thus, our attention has been attracted by the fact that the bond energy between a hydrogen atom and a carbon atom at the allyl position of an allyl structure is small, and we have carried out extensive studies on chain transfer agents for controlling the degree of polymerization in polymerizing vinyl monomers. As a result, it has been found that those compounds having an allyl oxy structure, --CH=CH--CH.sub.2 --O--, which forms a cyclic ether structure, have an excellent chain transfer effect. Furthermore, it has been found that these compounds neither hinder the polymerization reaction nor deteriorate the heat stability of a polymer formed and provide a polymer free of unpleasant odor.